The demand for tunable signal sources with low phase noise and a wide tuning range is growing, particularly tunable signal sources which can be used in higher frequency ranges, e.g. at 60 GHZ and above. Such a signal source can be used in a variety of technical fields such as, for example, radars and telecommunications.
One commonly used approach to obtain a high frequency signal source is to design a low frequency VCO, Voltage Controlled Oscillator, and to then apply frequency multipliers to up-convert the frequency of the VCO. Although it in this way is relatively easy to obtain an improvement in phase noise performance compared to a high frequency VCO, the phase noise degrades by 6 dB at every doubling of the oscillation frequency. This phase noise degradation, which accompanies an increase in frequency, can be alleviated by using a mixer and a fixed frequency oscillator in order to up-convert the frequency. Unfortunately, this approach has the drawback of having a relatively small tuning range, since the absolute tuning range determined by the low frequency VCO is unchanged in the frequency up-conversion.